I think they look at if they will be competitive and profitable on non-stops and if it's the best use of their planes. Sometimes it's not worth it for them to get into a "fare war", they may get more customers but are losing money at the low fare. I know they dropped non-stops from Detroit, where they were competing with Northwest. Boston already has Jet Blue, AirTran and Delta non-stops, they probably don't think they will be profitable jumping in too. Orlando does not have alot of business travel so it's hard for airlines to make money in that market (business travelers tend to make last minute reservations & are willing to pay higher fares).